1. Field of the Invention
The present invention relates to a steering control apparatus for a vehicle, particularly relates to an apparatus for controlling a steered wheel angle (tire angle) of a wheel to be steered, or applying a steering torque thereto, in response to steering operation of a vehicle driver, with respect to front or rear wheels of the vehicle to be steered.
2. Description of the Related Arts
In the United States Publication No. US2002/0013646 A1 (corresponding to Japanese Patent Laid-open Publication No. 2001-334947), for example, there is disclosed a motor vehicle steering system which is capable of controlling the attitude of a motor vehicle by controlling a steering mechanism. It is described in the Publication that in response to the detection of the actuation of the braking mechanism, the steering control circuit additionally turns the steerable wheels of the motor vehicle by a control steering angle toward one of the left and right wheels having a lower wheel speed on the basis of a result of judgement by the speed comparing circuit on condition that the speed difference between the left and right wheels exceeds the predetermined threshold value. With respect to a so-called “μ-split road”, it is explained that a road having significantly different friction coefficients with respect to left and right wheels of the motor vehicle. In that publication, the speed difference between the left and right wheels is employed as a reference for judging the “μ-split road”. And, a method for estimating a coefficient of friction of a road surface is described in the U.S. Pat. No. 6,447,076 B1 (corresponding to Japanese Patent Laid-open Publication No. 2000-108863).
According to the system as disclosed in the United States Publication No. US2002/0013646, it is so controlled that when the braking operation is performed on the μ-split road, the yaw moment acting on the motor vehicle at the initial stage of the braking operation is suppressed with a satisfactory responsiveness by the addition of the predetermined control steering angle for turning the front wheels toward the lower-speed wheel. In other words, by performing a so-called counter-steer control, the controlled yaw moment is applied in a reverse direction to the vehicle, to achieve a stability control of the vehicle. Although it is also described that the control angle can be set to be varied in response to braking force difference between the right and left wheels, the Publication No. US2002/0013646 is silent about its practical example.
As described above, in the case where the vehicle is running on a road surface with different coefficients of friction, with a pair of (right and left) wheels to be steered being positioned on the surface of different coefficients of friction from each other, respectively, if a braking operation is performed to each wheel to perform a so-called “μ-cross over braking”, it is required to perform an action properly reflecting the road surface condition. According to the system as disclosed in the United States Publication No. US2002/0013646, however, nothing has been evaluated about such a state as explained hereinafter.
In general, if lateral force is applied to the wheel to be steered, its slip angle can be obtained. According to the prior system, a μ-split road (D1) with a wet road surface having the coefficient of friction (μ) of 0.8, and an iced road surface having the coefficient of friction (μ) of 0.1, provides a difference of coefficient of friction (μ=0.7) between the split road surfaces. Also, a μ-split road (D2) with a dry road surface having the coefficient of friction (μ) of 1.0, and a pressed snow road surface having the coefficient of friction (μ) of 0.3, provides the same difference of coefficient of friction (μ=0.7) between the split road surfaces, as the one for the μ-split road (D1). Therefore, in the case where the road surface is determined only on the basis of the difference of braking force, i.e., it is determined by simply relying on the difference of coefficient of friction (μ), then, what type of the μ-split road can not be identified, as described later in detail with reference to FIG. 4. Therefore, even if the same lateral force was applied to the wheel, there would be caused a pretty large difference of the slip angle, which has resulted in one of the reasons for making an appropriate steering control operation to be difficult.